The purpose of the research supported herein is to select a few well documented examples of teratology and pursue these situations in great depth. One example we have chosen is the peculiar right foreleg ectrodactyly produced in rodent embryos by acetazolamide. This drug seems to act by inhibition of carbonic anhydrase and consequently we have examined the ontogeny, the distribution, the isozyme composition and inhibition kinetics in rodent embryos susceptible and resistant to the teratogenic effects of acetazolamide. We have found differences in all these factors between the embryos of sensitive and resistant mouse strains. The most pronounced differences have been found in embryonic erythrocytes which we now believe to be the basis of the drug-induced malformation. A second example has been our study of preaxial polydactyly of the hindlimb. We have demonstrated that alterations of physiological cell death within the limb are responsible for extra digit formation. Aspirin is one agent which causes this malformation and we have gone on to demonstrate that prostaglandins are found in the embryo during organogenesis and their inhibition lead to polydactyly and perhaps other malformations.